Recent progress of the electronic technologies is remarkable. In the operation of hydraulic cylinders for the working parts of construction equipment, electronic hydraulic control has replaced mechanical control. In the structure shown in, for example, FIG. 5, a converter 52 converts the control input of a lever 51 operated by the operator into an electric signal, and inputs it to a controller 53. The controller 53 outputs an electric signal, proportional to the control input to a solenoid 55 or 56 of an electro-magnetic proportional valve 54. The electro-magnetic proportional valve 54 is opened proportionally to the electric signal, and an amount of oil proportional to the amount of the opening is supplied to a working part cylinder 61 via pipe 58 and either pipe 59 or pipe 60. The discharged oil from the working part cylinder 61 is returned to a tank 63 via a pipe 62.
However, the above-described conventional technique is performed manually, and therefore simple and minute repeated operations of the hydraulic drilling machine, such as pressure-shifting or sifting work, make the operator exhausted. Also, in pressure-shifting, sifting or drilling carried out while the bucket is vibrated in order to increase the drilling force, the electro-magnetic proportional valve 54 must be activated at a very high speed. However, in an electro-magnetic proportional valve having a large capacity, the force of inertia is large and there is a limit to increasing the switching speed. Consequently, it is impossible to apply adequate vibration to the bucket to cope with changes in the operation form or the soil.
In view of the aforementioned problems, an object of the present invention is to provide an automatic vibration method and device for a hydraulic drilling machine which are capable of effective drilling by reducing the resistance to coping with various types of operations or changes in the soil.